A dual-layer equivalent channel method based on nonlinear Muskingum flood forecasting method is proposed to address the new challenges posed by the construction of national digital twin watersheds for predicting river water level and flow velocity in flood forecasting. This method uses the Muskingum parameters K and X to derive the formula for the dual-layer equivalent channel section, thereby achieving synchronous simulation of water level and flow velocity through hydrodynamic methods based on traditional flow calculation methods. Using the flood process from Chenggouwan Station to Linqing Station on the South Canal as an example for verification. The results show that compared with traditional method, the dual-layer equivalent channel method can not only forecast flow in areas lacking large cross-section data through hydrodynamic methods, but also simulate water depth and flow velocity information at any time and any cross-section of the river channel, achieving the expansion of large cross-section data of the river channel. The determination coefficients of the simulation results of the flow rate of Linqing Section using parabolic and rectangular dual-layer equivalent channel methods are both above 0.98, with peak error percentages of 0.3% and 0.17%, respectively, and root mean square errors of less than 20 m³/s. In terms of simulating the flow velocity of the Linqing Section, both the parabolic and rectangular dual-layer equivalent channel methods accurately simulated the time when the maximum flow velocity occurred.